1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image with an image sensor and generating image signals.
2. Related Background Art
In the conventional image reading apparatus, there are already known various methods for full-color image reading, such as by (1) light source switching, (2) color separation with prisms, (3) filter switching and (4) on-chip color filters, among which the on-chip color filter method is considered best in attaining high-speed reading and precise color separation.
FIG. 1 shows an example of the configuration of a conventional color CCD linear image sensor with the on-chip color filter method.
This conventional color CCD linear image sensor 1601 is a three-line image sensor composed of three CCD (charge coupled device) image sensor chips 1602-1604 arranged in parallel manner on a wafer and provided respectively with R, G and B on-wafer color filters.
A light-receiving unit 161 effects photoelectric conversion according to the amount of incident light (number being given only for the R unit but G and B units are constructed similarly). On the CCD sensor elements of these light-receiving units 161, there are provided R, G and B on-wafer color separation filters. At an end of the light-receiving unit 161 an aluminum mask is provided on the light-receiving unit 161 to intercept the incident light, thereby forming a light-shielded pixel for constantly providing an output corresponding to a dark state. Transfer gates 162, 163 transfer the charges, accumulated in the light-receiving unit 161, to CCD shift registers 164, 165 in response to a shift gate pulse .phi..sub.TG. The charges accumulated in the even-numbered pixels of the light-receiving unit 161 are transferred through the transfer gate 163 to the CCD shift register 165 for the even-numbered pixels, while those in the odd-numbered pixels of the light-receiving unit 161 are transferred through the transfer gate 162 to the CCD shift register 164 for the odd-numbered pixels.
The CCD shift registers 164, 165, for effecting CCD transfer (complete transfer) of the charges, transferred from the light-receiving unit 161, to output units, are two-phase driven by drive clock signals .phi..sub.1 (.phi..sub.1R, .phi..sub.1FR, .phi..sub.1G, .phi..sub.1FG, .phi..sub.1B, .phi..sub.1FB and .phi..sub.2 ( .phi..sub.2R, .phi..sub.2FR, .phi..sub.2G, .phi..sub.2FG, .phi..sub.2B, .phi..sub.2FB).
An output gate 166 serves to transfer the charges from the CCD shift registers 164, 165 respectively to output capacitors 167a, 167b which convert the transferred charges into voltages. Two-stage source follower amplifiers 168a, 168b serve to reduce the output impedance, thereby eliminating noise from the output signal.
The output capacitors 167a, 167b and the source follower amplifiers 168a, 168b constitute floating diffusion amplifiers (FDA).
There are also provided signal output terminals OSAR, OSBR, OSAG, OSBG, OSAB, OSBB; reset pulse terminals .phi.RAR, .phi.RBR, .phi.RAG, .phi.RBG, .phi.RAB, .phi.RBB; CCD shift register clock terminals .phi.1R, .phi.1G, .phi.1B, .phi.2R, .phi.2G, .phi.2B; transfer gate clock terminals .phi.TGR, .phi.TGG, .phi.TGB; and source follower amplifier drain terminals ODR, ODG, ODB.
In the color image sensor 1601 of the above-explained configuration, the light falling onto the light-receiving unit 161 is converted into charges proportional to the amount of light. These charges in the even- and odd-numbered pixels are respectively transferred to the CCD shift registers 164, 165 in response to the shift gate pulse .phi..sub.TG, then output bit by bit to the FDA through the output gate 166 according to the drive clock signals .phi..sub.1, .phi..sub.2, then converted into voltages by the output capacitors 167a, 167b of the FDA, and finally output through the two-stage source follower amplifiers 167a, 167b and the output terminals OSA, OSB.
However, the conventional configuration explained above shows overall spectral sensitivity characteristics as shown in FIG. 5, because of spectral transmittances of the R, G and B filters shown in FIG. 2, a spectral energy distribution of the light source shown in FIG. 3 and a spectral transmittance of an infrared absorbing filter shown in FIG. 4, whereby the amounts of charges generated in the photodiodes of the CCD's 1602, 1603, 1604 are in the order of B-CCD &lt; R-CCD &lt; G-CCD. Eventually the CCD's 1602, 1603, 1604 have the sensitivities increasing in the order of B-CCD &lt; R-CCD &lt; G-CCD, so that the sensitivities of the CCD's become, for example, 2.1 V/1.times..sec for R, 2.6 V/1.times..sec for G and 0.86 V/1.times..sec for B.
The saturated output voltage is usually the same for the CCD's 1602, 1603 and 1604 because the CCD shift registers 164, 165 are of a same size.
In the actual use of such 3-line color CCD linear sensor, the structure of the image reading system is determined by the amount of light which gives an output voltage providing a necessary S/N ratio.
Stated differently, it is determined by the amount of light when the output voltage of the B-CCD 1604 of the lowest sensitivity provides the required S/N ratio. For example, for a required S/N ratio of 48 dB (256 levels) and a noise level of 1 mV from the CCD, the minimum output voltage becomes 256 mV.
Consequently, when the output voltage of the B-CCD is 256 mV, the output voltages of the R-CCD 1602 and the G-CCD 1603 respectively become:
2.1 (V/1.times..sec)/0.86 (V/1.times..sec).times.256 mV&lt;625 mV; PA1 2.6 (V/1.times..sec)/0.86 (V/1.times..sec).times.256 mV&lt;774 mV.
Assuming that the above-mentioned output voltages are obtained for a certain accumulation time T.sub.1 (.mu.sec), there is required an accumulation time T.sub.2 =T.sub.1 /2 for doubling the reading speed of the image reading device. In such case the output voltages of the CCD's become 128 mV for B-CCD, 325 mV for R-CCD and 383 mV for G-CCD, so that the S/N ratio of 48 dB cannot be secured for the B-CCD 1604.
For compensating such drawback, the amount of light has to be doubled, but a mere doubling of the light amount cannot solve the problem for example because of the temperature rise in the device. For successful designing of the device, it is necessary to increase the amount of illuminating light to an extent that will not causing the temperature rise and to cover the deficiency by sacrificing the S/N ratio of the B-CCD. Consequently there inevitably results a deterioration in the image quality.
For this reason there is proposed a linear image sensor of so-called TDI (time delay and integration) system, of the configuration shown in FIG. 6. Such TDI linear image sensor is provided with photoelectric conversion means in plural lines, of which output signals are synthesized in succession, in synchronization with the reading speed of the scanner employing such line sensor, thereby providing an output signal of several times of that of each line of the photoelectric conversion means in the line sensor.
In FIG. 6, the color CCD linear sensor 1700 capable of the above-explained TDI operation is composed of red, blue and green CCD linear sensors 1701, 1702, 1703.
1704a-1704c indicate linear photodiode arrays with red on-chip color filters, and 1705a and 1705b indicate CCD shift registers for horizontally transferring the charges, generated in the photodiode arrays 1704a-1704c, to output units 1718a, 1718b. The two CCD shift registers 1705a, 1705b are used for increasing the reading speed of the color CCD linear image sensor.
Similarly, there are provided linear photodiode arrays 1706a-1706c, 1708a-1708c respectively provided with blue and green on-chip color filters; CCD shift registers 1707a, 1707b; 1709a, 1709b respectively for blue and green signals; and output units 1719a, 1719b; 1720a, 1720b respectively for the CCD shift registers for the blue and green signals.
There are also provided shift gates SH1 (1710), SH3 (1712) for temporarily accumulating the charges, generated in the linear photodiode arrays, in synchronization with the reading speed of the scanner, and shift gates SH2 (1711), SH4 (1713) for transferring the charges, accumulated in the shift gates SH1 (1710), SH3 (1712), therefrom to linear photodiode arrays 1704b, 1704c of the next stage, for synthesizing the charges generated therein.
A shift gate SH5 (1714) transfers the charges, generated in the linear photodiode array 1704c, to the CCD shift registers 1705a-1705d in synchronization with the reading speed of the scanner, and the charges transferred from the linear photodiode array 1704c through the shift gate SH5 (1714) are transferred, pixel by pixel, to the CCD shift registers 1705a, 1705b through the respectively corresponding pixel elements of shift gates SG1, SG2 (1715-1718).
Also a transfer gate TG1 (1717) is provided for charge transfer between the CCD shift registers 1705a and 1705b.
The CCD linear sensors 1702, 1703 for blue and green colors are constructed similarly to the CCD linear sensor 1701 for red color and will not, therefore, be explained further.
In the color CCD linear sensor 1700, as explained in the foregoing, the integration of the charges and the transfer of the charges between the photodiode arrays and the CCD shift registers or between the CCD shift registers are effected only in a direction indicated by arrows.
FIG. 7 is a view showing an example of the configuration of a scanner 1800 equipped with the color CCD linear sensor 1700 shown in FIG. 6.
The scanner 1800 is composed of a scanner main body 1800a and a document feeder 1800b.
There are also shown a platen glass 1810 for supporting an original, a halogen lamp 1805 for illuminating the original, and a first mirror 1802, the latter two constituting a mirror unit 1812.
A second mirror 1803 and a third mirror 1804 constitutes another mirror unit 1813.
A lens unit 1801 focuses, with size reduction, the light reflected from the original scanned by the halogen lamp 1805, onto the color CCD linear sensor 1700. A running-reading platen glass 1809 is provided for running reading of the originals with the document feeder 1800b.
In case of reading the original by placing it on the platen glass 1810 and moving the mirror units 1812, 1813 in a direction A (sub scanning direction) with a speed ratio of 2:1 by the stepping motor 1814, the mirror units 1812, 1813 start from the broken-lined positions.
The document scanner 1800b has the following structure.
There are provided an original input tray 1806, an original pickup roller 1807, an original feed rollers 1808, and an original discharge tray 1811.
The originals are stacked, with the top sides thereof facing upwards, on the input tray 1806. In case of one-side reading, the original is advanced by the pickup roller 1807 to the feed rollers, then fed by the feed rollers in the direction of a broken arrow according to the predetermined timing of original reading, and is subjected to running reading upon passing on the running-reading platen glass, whereby the reflected image is focused, in reduced size, onto the color CCD linear sensor 1700 through the mirror units 1812, 1813 and the lens unit 1801.
In case two-side reading of the original, the original is transported by the feed rollers in the direction indicated by solid-lined arrows, thus subjected to the reading of the top side in passing the reading position on the running-reading platen glass, then is inverted along a transport path for reading the bottom side in a movement opposite to that in the top side reading, and is discharged to the tray 1811 in a similar manner as in the case of one-side reading.
In such operation, the image focused on the color CCD linear sensor 1700 is scanned in a direction B or C respectively in the top side reading or in the bottom side reading. As the color CCD linear sensor 1700 shown in FIG. 5 can effect the TDI integration only in one direction, the image reading can be achieved only for the top side or the bottom side, depending on the mounting direction of the linear sensor 1700. For this reason there is further proposed a color CCD linear sensor 1300 as shown in FIG. 8, composed of CCD linear sensor units 1301, 1302, 1303 respectively for red, blue and green colors.
There are provided linear photodiode arrays 1301a-1301c, constituting photoelectric conversion means and provided with red on-chip color filters; and similar linear photodiode arrays 1303a-1303c, 1304a-1304c provided with blue and green on-chip color filters.
There are also provided CCD shift registers 1305a, 1305b; 1308a, 1308b for horizontally transferring the charges, generated in the linear photodiode arrays 1301a-1301c, to output units 109a, 109b; 130a, 130b, wherein the shift registers 1305a, 1305b are for forward (top side) reading (indicated by solid-lined arrow in FIG. 8), while those 1308a, 1308b are for backward (bottom side) reading (indicated by broken-lined arrow in FIG. 8).
The CCD shift registers 1305a, 1305b are also used for horizontally transferring the charges, generated in the arrays 1303a-1303c, to the output units 109a, 109b (for backward reading of the blue color).
CCD shift registers 1306a, 1306b are provided for transferring the charges, generated in the linear photodiode arrays 1303a-1303c for blue color, to output units 110a, 110b and are used for forward reading of blue color.
They are also used for outputting the charges generated in the linear photodiode arrays 1308a-1308b for green color (for backward reading of green color).
CCD shift registers 1307a-1307b are provided for horizontally transferring the charges, generated in the linear photodiode arrays 1304a-1304c for green color, to output units 111a, 111b.
Shift gates SH1 (112), SH2 (113), SH3 (114), constituting first charge transfer means, are provided for transferring the charges generated in the linear photodiode array 1301a, to the next linear photodiode array 1301b for synthesizing with the charges generated therein. In the forward reading, the shift gates SH1 (112), SH2 (113) and SH3 (114) are activated in succession to transfer the charges in the direction indicated by the solid-lined arrow, but, in the backward reading, they are activated in the reverse order, i.e. SH3 (114) .fwdarw. SH2 (113) .fwdarw. SH1 (112).
Also shift gates SH4-SH6 (115-117), constituting first charge transfer means, are provided for vertically transferring the charges in order to synthesize the charges generated in the linear photodiode arrays 1301b and 1301c, and they are activated in an order of SH4 .fwdarw. SH5 .fwdarw. SH6 or SH6 .fwdarw. SH5 .fwdarw. SH4 respectively in the forward or backward reading.
A shift gate SH7 (118) transfers the charges generated in the linear photodiode array 1301c to the horizontal CCD shift registers 1305a, 1305b in synchronization with the reading speed of the scanner. Switch gates SG1 (119), SG2 (120) transfer the charges of the linear photodiode array 1301c, transferred by the shift gate SH7 (118), pixel by pixel to the horizontal CCD shift registers 1305a, 1305b. The switch gate SG1 transfers the charges of the odd-numbered pixels to the shift register 1305b, and the switch gate SG2 transfers the charges of the even-numbered pixels to the shift register 1305a.
Transfer gates TG1-TG3 (121-123), constituting second charge transfer means, transfer charges between the horizontal CCD shift registers 1305a, 1305b. As in the above-explained charge transfer between the linear photodiode arrays, the direction of transfer can be switched by changing the order of functions of these transfer gates as TG1 .fwdarw. TG2 .fwdarw. TG3 or TG3 .fwdarw. TG2 .fwdarw. TG1 (solid-lined arrow indicating forward direction, and broken-lined arrow indicating backward direction).
The horizontal CCD shift registers 1305a, 1305b are of two-phase drive, and are composed of alternate connection of .phi.1 and .phi.2 registers as already known, and are adapted to transfer the charges in succession toward the output units (109a, 109b) by the change in the potential of the VVD register, through alternate pulse inputs to the two registers. In the above-explained charge transfer by the transfer gates TG1-TG3, there are only used the .phi.1 registers among the two registers.
A shift gate SH8 (124) transfers the charges of the linear photodiode array 1301a to the CCD shift registers 1308a, 1308b in synchronization with the reading speed of the scanner. Switch gates SG1 (125), SG2 (126) transfer the charges of the shift gate SH8 pixel by pixel to the CCD shift registers 1308a, 1308b. The switch gate SG1 transfers the charges of the odd-numbered pixels to the CCD shift register 1308a, while the switch gate SG2 transfers the charges of the even-numbered pixels to the CCD shift register 1308b.
Transfer gates TG1-TG3 (127-129) effect charge transfer between the CCD shift registers 1308a and 1308b in a manner similar to that explained before.
FIGS. 9A and 9B are timing charts showing the function of the color CCD linear image sensor 1300 shown in FIG. 8.
In case of forward reading shown in FIG. 9A, signals SH7, SG1 and SG2 are shifted to "H" at a timing T.sub.1, whereby the charges of the linear photodiode array 1301c are transferred to the shift gates SG1 (119), SG2 (120). Then, at T.sub.2, SG1 is shifted from "H" to "L" and .phi.1 is shifted from "L" to "H", whereby the charges of the shift gate SG1 (119) are transferred to the .phi.1 registers of the CCD shift register 1305a.
Then at T.sub.3, .phi.1 is shifted from "H" to "L" and TG1 assumes "H", whereby the charges are transferred from the .phi.1 registers to the transfer gate TG1 (121).
Similarly, TG1 is shifted from "H" to "L" while TG2 is shifted from "L" to "H" at T.sub.4, and TG2 is shifted from "H" to "L" while TG3 is shifted from "L" to "H" at T.sub.5, whereby the charges are transferred in succession from the transfer gate TG1 (121) to TG2 (122) and then to TG3 (123).
Then, at a timing T.sub.6, the switch gate SG2 (120) is shifted from "H" to "L" and .phi.1 is shifted again from "L" to "H", whereupon the gates of the switch gate SG2 (120) are transferred to the .phi.1 registers of the CCD shift register 1305a, and the transfer gate TG3 (120) is shifted from "H" to "L" whereby the charges thereof are transferred to the .phi.1 registers of the CCD shift register 1305b.
The transfer operation in the shift gates SH1-SH7 for the TDI operation is conducted, as illustrated, by a shift gate in each cycle, from SH1 to SH7, and the charges are transferred to the CCD shift registers 1305a, 1305b in the 7th cycle and are output.
In case of backward reading shown in FIG. 9B, the operations are similar to those in the forward reading except that the shift gate SH7 (118) is replaced by the shift gate SH8 (124), and that the timings of function of the shift gates SH1-SH6 (112-117) and of the transfer gate s TG1-TG3 (121-123) are inverted.
The foregoing explanation has been limited to the CCD linear image sensor unit 1301 for red color in the color CCD linear image sensor 100, but the functions of the CCD linear image sensor units 1302, 1303 for blue and green colors are similar and will not, therefore, be explained further. The signals thus obtained are output from a CPU not shown.
However, in such color CCD linear image sensor provided with plural linear sensor arrays for adding the output charges thereof in synchronization with the timing of signal reading thereby increasing the output signal by the number of the sensor arrays, and so constructed as to effect the charge integration in forward and backward directions and to commonly utilize the CCD shift registers and the output units between different colors for enabling image reading both in the forward and backward directions, the CCD output signal obtained from each output unit of the color CCD linear image sensor represents different colors in the forward and backward readings, with consequently different output levels.